Carbides and nitrides, represented by silicon carbide and silicon nitride, possess excellent properties such as hardness, thermal conductivity, heat resistance, thermal shock resistance, and chemical stability. They are being used increasingly as grinding and polishing materials, fire resistant materials, heating elements, electrical parts, and disoxidants. They are produced by calcining a mixture of a material powder and carbon under a reducing atmosphere or nitrogen gas atmosphere.
Silicon carbide is at present prepared as follows.
Alpha-silicon carbide powder is generally produced by filling a mixture of silica and coke in an electric resistant furnace, heating the mixture with application of electricity, and grinding and classifying the obtained crystalline lump.
To produce beta-silicon carbide, the following five methods have been employed.
(1) A direct reaction of silicon with carbon: A mixture of silicon powder and fine carbon powder such as carbon black is calcined at a temperature in the range of 1000.degree.-1400.degree. C. PA0 (2) Reduction reaction of silica with carbon: A mixture of silica powder and fine carbon powder is calcined at a temperature in the range of 1500.degree.-1800.degree. C. under an inert atmosphere. This method is being employed in an industrial scale as a stable method for continuous mass production with use of a vertical reaction furnace. PA0 (3) Vapor phase reaction: Hydrocarbon is reacted with silicon tetrachloride or silicon tetrahydrogenate, or varying alkylsilanes are subjected to thermal decomposition. This method is suitable to produce micronized powder of a particle size of less than 0.1 .mu.m having high purity and less aggregation. It, however, has various drawbacks, for example, that the material gas is expensive and the used apparatus is corroded by chlorides. PA0 (4) Gas evaporation: The raw materials are heated at high temperature to be evaporated and subjected to a reaction, and the prepared fine particles are aggregated. It has been studied to employ a method of directly passing electricity or making an arcing between a carbon rod and silicon. This method has not been put in practice. PA0 (5) Thermal decomposition of organic silicon polymer: An organic silicon polymer represented by polycarbosilane is thermally decomposed at a temperature more than 1500.degree. C. under a non-oxidizing atmosphere. Since the material organic silicon polymer is quite expensive, this method is not so suitable for the industrial production of beta-silicon carbide powder. PA0 (1) A prescribed organic compound and solvent are intercalated in advance into the host interlamellar spaces of a lamellar structured substance in order to enlarge the interlamellar spaces, so that a guest reaction substance can be easily entered into these spaces. PA0 (2) An inorganic or organic guest reaction substance is intercalated into the enlarged host interlamellar spaces, so that the guest reaction substance contacts with the crystal faces of every layer of the host crystal and the reaction can be conducted microscopically. PA0 (3) The guest reaction substance intercalated into the host interlamellar spaces is polymerized or condensed, so that the guest reaction substance is directly reacted with the host crystal faces without suffering evaporation of the reaction substance until its temperature is raised high. PA0 (4) The guest reaction substance is converted into a high-molecular compound and subjected to a flame resistant treatment at a temperature in the range of 200.degree.-250.degree. C. This treatment allows the intercalated guest reaction substance to react until its temperature rises high without suffering evaporation and eliminates the organic substance which is used to enlarge the interlamellar spaces of the host crystal. Since the decomposition product enhances the reducing atmosphere, the reaction readily proceeds, the reaction temperature is lowered, the yield is increased, and the crystallinity is increased.
As described above, silicon carbide is at present industrially produced by simply mixing material powders macroscopically and calcining the prepared mixture under a reducing atmosphere or an inert atmosphere. Other carbides or nitrides are prepared in the form of powder through a macroscopical contact of powder solids although the vapor phase reaction is employed when they are produced in the form of whisker. It may be summarized from the above that synthesis of carbide or nitride powder depends on a reaction among solid particle surfaces of a material mixture caused by calcination of the mixture at high temperature, or a reaction of the sublimed gaseous components or a thermal decomposition reaction thereof. Consequently, the reaction must be carried out at amply high temperature under a reducing atmosphere or inert atmosphere to satisfactorily produce nitrides and carbides.